The invention relates to a process and a device for carrying out a process for the continuous production of, in particular, polyamide 6, polyamide 6.6 and copolyamides from .epsilon.-caprolactam and/or a salt of adipic acid with hexamethylenediamine or adipic acid and hexamethylenediamine. It is known that the production of polyamide 6, polyamide 6.6 and copolyamides should be carried out in sequences specially adapted in each case to the requirements of the product and with an arrangement of apparatus tailored to an optimal process sequence.
The polymerization of .epsilon.-caprolactam involves hydrolysis, polyaddition and polycondensation reactions whose most advantageous reaction rates occur in each case under other conditions. The hydrolysis of .epsilon.-caprolactam to .epsilon.-amino-caproic acid is the first and slowest reaction of the polymerization process. Expediently, the hydrolysis is carried out at high temperatures and with high water contents of the reaction mixture in the first process stage, in order to utilize the maximum possible hydrolysis rate. This process has to be carried out in a pressure stage. A low-viscosity product is formed. In this pressure stage, a viscosity limit is usually imposed by the added amount of water. In order to achieve the viscosity required for the processing of the polymer, the water added in excess to the pressure stage is expelled in one or more subsequent process stages operated at atmospheric pressure.
The desired viscosity is established by adding chain terminators at the beginning of the polymerization process and by a specific temperature program in the individual reaction steps (East German patent 133 681).
For the polymerization of a salt of adipic acid with hexamethylenediamine or adipic acid and hexamethylenediamine, it is known that an aqueous AH salt solution, which is heated in a pressure reactor to a temperature in the range of 220-280.degree. C., is used as a starting material, with the aim of preparing an AH salt precondensate in which the NH.sub.2 groups of the hexamethylenediamine are virtually all reacted with the COOH groups of adipic acid. It is also known that the aqueous hexamethylenediamine/adipic acid salt solution is heated under pressure with simultaneous evaporation of the water, and the expelled diamine is retained in a column and is recycled to the polymerization process. When such a precondensate is let down to atmospheric pressure, hexamethylenediamine can scarcely escape and no viscosity-reducing excess of acid is obtained. Dewatering and post-condensation of the polyamide 6.6 melt is carried out in principle as for the production of polyamide 6. The dewatering stage should however be adjusted and dimensioned for substantially more extensive dewatering (U.S. Pat. No. 2,689,839; German Offenlegungschrift 2,401,474). Known processes for the production of copolyamides from aqueous AH salt solution and .epsilon.-caprolactam assume that the production will be carried out in barometric reactors, so-called precondensation tubes. In these processes, however, only a maximum of 8% of AH salt can be added to the .epsilon.-caprolactam. A procedure using a high water content, owing to the aqueous AH salt solution which as a rule has a solids content of from 50 to 65% by weight, is difficult to control and, with additions greater than 8% of AH salt solution, leads to considerable quality fluctuations through nonuniform evaporation of water and, in spite of a rectification column on the precondensation tube, different amounts of hexamethylenediamine are discharged. Precondensate formed in a pressure stage and comprising a hexamethylenediamine/adipic acid salt solution is therefore mixed with molten caprolactam in a downstream, barometrically operated reactor, and a copolyamide is obtained as the polymerization progresses. However, this process permits the addition of .epsilon.-caprolactam to the aqueous AH salt solution only in amounts of up to 50% (German Offenlegungschrift 3,912,768).
The disadvantage of this process is that in this case:
a precondensate of higher viscosity is mixed with a low-viscosity product, .epsilon.-caprolactam, leading to a nonuniform polymer, PA0 only 50% of .epsilon.-caprolactam can react with an aqueous AH salt solution, whereas lactam additions between 75 and 98% are of interest for the polyamide processor. PA0 a) preheating of the starting materials in a common heat exchanger, and PA0 b) polymerization of the starting materials in a pressure stage with or without expulsion of water, by a procedure in which, with water expulsion in a rectification column, in each case the caprolactam or the diamine or, in the production of the polyamide, both these products are recycled to the pressure stage, PA0 c) dewatering of the polymeric melt in a coiled tube which comprises one to five turns and in which the polymeric melt is subjected to a specific, slow pressure reduction, this coiled tube being designed in such a way that both the high-viscosity polyamide 6 prepolymer and the low-viscosity polyamide 6.6 precondensate under a higher pressure as well as the copolymer are transported or forced without problems into the first postcondensation reactor operated at virtually barometric pressure, PA0 d) first postcondensation in a common, vertical reactor at temperatures in the range 210-285.degree. C. and with a process sequence from top to bottom, by forming thin melt films and expelling the water no longer required for the further polymerization process, and PA0 e) further postcondensation of the melt in downstream reactors if high viscosities are to be achieved.
European patent application 0 393 546 describes a process for the continuous production of copolyamides, an aqueous solution of diamines and dicarboxylic acids being subjected to a precondensation under pressure and with evaporation of water. In a further process stage, the vapor phase is separated from the prepoymer melt. This melt is mixed with caprolactam and fed in a vertical tube to the polymerization process in order thus to obtain a copolyamide.
The prior art is such that a specially developed reactor type is used for each polymer. This disadvantage is to be overcome by the invention.